This invention relates, in general, to radioactivity well logging, and more particularly to methods and apparatus for determining the macroscopic thermal neutron capture cross-section of the formations surrounding a borehole as determined by radiation measurements.
It is known in the art of radioactivity well logging, for example, as illustrated and described in U.S. Pat. No. 4,046,764 issued to Gerald L. Marquis, which is assigned to the assignee of the present invention, to establish the points in time of which radiation is detected within a time interval beginning at a fixed time following each neutron burst from a neutron source. A unique solution is obtained where each measured value of the time relationship of the detected pulses defines a single value of macroscopic neutron absorption cross-section (Sigma (.SIGMA.)). In the prior art, the points in time of the detected radiation pulses are determined by using a plurality of pulse registers and a corresponding number of gate circuits. This technique requires unduly complex processings for the calculation of the measured value of the time relationship and the conversion of that value to the macroscopic neutron absorption cross-section. Further, should more detected pulses be present within a given measure interval than the number of pulse registers or gate circuits available these additional pulses will not be processed. Additionally, should one of the plurality of pulse registers or plurality of gate circuits become inoperative, there is no apparent indicator of the failure of the circuitry, resulting in an erroneous Sigma calculation.
Accordingly, the present invention overcomes the deficiencies of the prior art by providing circuitry for establishing the exact points in time of which a preselected number of radiation pulses are detected within measurement intervals following neutron bursts from a neutron source and obtaining from these points in time a signal indicative of the mean time to be used for determining the macroscopic thermal neutron capture cross-section of the formation surrounding the borehole.